I. Bone Loss
The current major diseases or conditions of bone which are of public concern include post-menopausal osteoporosis, ovariectomy patients, senile osteoporosis, patients undergoing long-term treatment of corticosteroids, side effects from glucocorticoid or steroid treatment, patients suffering from Cushings's syndrome, gonadal dysgensis, periarticular erosions in rheumatoid arthritis, osteoarthritis, Paget's disease, osteohalisteresis, osteomalacia, hypercalcemia of malignancy, osteopenia due to bone metastases, periodontal disease, and hyperparathyroidism. All of these conditions are characterized by bone loss, resulting from an imbalance between the degradation of bone (bone resorption) and the formation of new healthy bone. This turnover of bone continues normally throughout life and is the mechanism by which bone regenerates. However, the conditions stated above will tip the balance towards bone loss such that the amount of bone resorbed is inadequately replaced with new bone, resulting in net bone loss.
One of the most common bone disorders is post-menopausal osteoporosis which affects an estimated 20 to 25 million women in the United States alone. Women after menopause experience an increase in the rate of bone turnover with resulting net loss of bone, as circulating estrogen levels decrease. The rate of bone turnover differs between bones and is highest in sites enriched with trabecular bone, such as the vertebrae and the femoral head. The potential for bone loss at these sites immediately following menopause is 4-5% per year. The resulting decrease in bone mass and enlargement of bone spaces leads to increased fracture risk, as the mechanical integrity of bone deteriorates rapidly.
At present, there are 20 million people with detectable vertebral fractures due to osteoporosis and 250,000 hip fractures per year attributable to osteoporosis in the U.S. The latter case is associated with a 12% mortality rate within the first two years and 30% of the patients will require nursing home care after the fracture. Therefore, bone disorders are characterized by a noticeable mortality rate, a considerable decrease in the survivor's quality of life, and a significant financial burden to families.
Essentially all of the conditions listed above would benefit from treatment with agents which inhibit bone resorption. Bone resorption proceeds by the activity of specialized cells called osteoclasts. Osteoclasts are unique in their ability to resorb both the hydroxyapatite mineral and organic matrix of bone. They are identical with the cartilage resorbing cells, previously termed chondroclasts. It is for this reason that potent inhibitors of osteoclastic bone resorption will also inhibit the cell-mediated degradation of cartilage observed in rheumatoid arthritis and osteoarthritis.
Therapeutic treatments to impede net bone loss include the use of estrogens. Estrogens have been shown clearly to arrest the bone loss observed after menopause and limit the progression of osteoporosis; but patient compliance has been poor because of estrogen side-effects. These side effects include resumption of menses, mastodynia, increase in the risk of uterine cancer, and possibly an increase in the risk of breast cancer.
Alternatively, calcitonin has been used to treat osteoporotic patients. Salmon calcitonin has been shown to directly inhibit the resorption activity of mammalian osteoclasts and is widely prescribed in Italy and Japan. However, calcitonins are prohibitively expensive to many and appear to be short-lived in efficacy. That is, osteoclasts are able to "escape" calcitonin inhibition of resorption by down-regulating calcitonin receptors. Therefore, recent clinical data suggest that chronic treatment with calcitonin may not have long term effectiveness in arresting the post-menopausal loss of bone.